1. Field of the Invention
The present invention relates to a method for fabricating a metal insulator semiconductor type semiconductor device, i.e. a MIS type semiconductor device (also known as an insulated gate type semiconductor device). The MIS type semiconductor device includes, for example, a MOS transistor, thin film transistor and the like.
2. Description of the Related Art
Conventionally, a MIS type semiconductor device has been fabricated by using a self-aligning method. According to this method, a gate wiring (electrode) is formed on a semiconductor substrate or semiconductor coating film via a gate insulating film and impurities are injected into the semiconductor substrate or semiconductor coating film using the gate wiring as a mask. Such methods as a heat diffusion method, ion injection method, plasma doping method and laser doping method are used as means for injecting the impurities. Such means allow an edge of the gate electrode to substantially coincide with that of the impurity region (source and drain) and to eliminate an overlap state (a cause of parasitic capacity) in which the gate electrode overlaps the impurity region and an offset state (a cause of effective mobility degradation) in which the gate electrode is separated from the impurity region.
However, the conventional process has a problem in that spatial changes of carrier density in the impurity region and in an active region (a channel forming region) which adjoins with the impurity region and is located under the gate electrode are too large, so that a remarkably large electric field is created and in particular, a leak current (OFF current) increases when an inverse bias voltage is applied to the gate electrode.
The inventors of the present invention found that this problem could be alleviated by slightly offsetting the gate electrode and the impurity region. Then the inventors formed the gate electrode by an anodizable material in order to realize this offset state. As a result of the anodization, they found that a constant size offset state could be obtained with good reproducibility by injecting impurities using an anodized film produced as a mask.
Further, because the crystallinity of a semiconductor substrate or semiconductor coating film is damaged in parts where ions have penetrated in methods such as the ion injecting method and plasma doping method, by which impurities are injected by radiating high speed ions onto the semiconductor substrate or semiconductor coating film, improvement (activation) of the crystallinity is required. Although the crystallinity has been improved mainly thermally at a temperature of over 600.degree. C. heretofore, a tendency towards a low temperature process has recently become apparent. Then the inventors also showed that activation can be implemented by emitting laser beams or strong light equivalent thereto, and that the massproducibility thereof is excellent.
FIG. 2 shows a process for fabricating a thin film transistor based on the aforementioned concept. Firstly, a ground insulating layer 202 is deposited on a substrate 201 and then an island crystalline semiconductor region 203 is formed. Then an insulating film 204 which functions as a gate insulating film is formed thereover. A gate wire 205 is then formed using an anodizable material (FIG. 2A).
Next, the gate wire is anodized and an anodic oxide 206 having a suitable thickness of less than 300 nm or preferably less than 250 nm for example is formed on the surface of the gate wire. Then using this anodic oxide as a mask, impurities (for example phosphorus (P)) are emitted in a self-aligning manner by means of ion injection or ion doping to form impurity regions 207 (FIG. 2B).
After that, the regions into which the impurities have been injected are activated by radiating a strong light such as a laser beam thereon from above (FIG. 2C).
Finally, the thin film transistor is completed by depositing an interlayer insulator 208 and by creating contact holes on the impurity regions to create electrodes 209 to be connected to the impurity regions (FIG. 2D).
However, it was found that according to the method described above, the physical property at a boundary (indicated by X in FIG. 2C) between the impurity region and an active region (a semiconductor region sandwiched between the impurity regions directly under the gate electrode) is unstable and that problems could arise such as a leak current increasing over time and reliability decreasing. That is, as seen from the process, the crystallinity of the active region does not substantially change from the beginning. On the other hand, although the impurity region which adjoins the active region initially has the same crystallinity as the active region, its crystallinity is destroyed in the process of injecting a large quantity of impurities (up to 10.sup.15 cm.sup.-2). It was then found that, although the impurity region is restored in the latter process of radiating laser beams, it is difficult to reproduce the same crystalline state as the original one and that the portion of the impurity region which contacts the active region in particular tends to be shaded during irradiation by laser beams and cannot be fully activated.
That is, the crystallinity of the impurity region and the active region is discontinuous and, due thereto, a trap level or the like tends to be produced. Especially when the method of radiating high speed ions is adopted as a method for injecting impurities, the impurity ions wrap around under the gate electrode section due to scattering and destroy the crystallinity of that portion. Then it is impossible to activate the region under the gate electrode section by laser beams or the like because it is shadowed by the gate electrode section.
One method for solving this problem is to activate that portion by radiating light such as a laser beam thereon from behind. This method fully activates the boundary between the active region and the impurity region because it is not shadowed by the gate wire. However, the substrate material must transmit the light in this case and, as a matter of course, this method cannot be employed when a silicon wafer or the like is used. Furthermore, because many glass substrates do not transmit ultraviolet light of less than 300 nm, a KrF excimer laser (wavelength: 248 nm), for example, which has excellent massproducibility, cannot be utilized.
Accordingly, it is an object of the present invention to solve the aforementioned problems and to obtain a highly reliable MIS type semiconductor device such as a MOS transistor and thin film transistor by achieving continuity between the crystallinity of the active region and the impurity region.